Both institutional and consumer automatic dishwashers or warewashing machines have been in use for many years. These dishwashers typically function with two or more cycles, including various combinations of a soak or prewash, a main wash, a rinse, and a high temperature sanitizing rinse cycle. A dishwasher detergent composition is typically utilized during the wash cycle to remove soil and stains. Often, the detergent composition will include water softeners or sequestrants, bleaching and sanitizing agents, and an alkali source. Dishes and other wares washed in automatic washing machines are preferably obtained without food soils and without residue from the cleaning solutions or other chemicals used in the washing process.
One type of residue, known as streaking and spotting, is common on machine washed dishes. Streaking and spotting is believed to result when water salts deposit on the dishes after the rinse cycle drainage and evaporation. Rinse additives or aids are commonly added to rinse water in an effort to reduce surface tension of the rinse water and thereby promote sheeting of the water from the dishes. Typical rinse aid formulas require solution concentrations ranging from about 250 ppm to 1,000 ppm (depending on actives) to provide efficient sheeting and drying.
For many reasons, separate rinse additives or aids are an important part of the automatic dishwasher operation. In general, rinse aids minimize spotting and promote faster drying by causing the rinse water to sheet off of the clean dishes and other wares evenly and quickly. Rinse aids are generally used in a cycle separate from cycles using the detergent composition, although some detergent residue may be present in the rinse water. Rinse aids are currently available in liquid or solid form.
Solid rinse aids are generally more convenient, safe and economical than liquid rinse aids because they do not spill or splash, have reduced manufacturing and distribution costs, and require less storage space. Solid rinse aids are available for consumer and institutional warewashing machines. For use in a typical consumer wash machine, each solid rinse aid generally incorporates a disposable container or basket which is hung directly inside the machine. Circulation of water within the wash machine in the normal course of the machine cycles slowly dissolves the solid rinse aid, thus dispensing it. The water temperature in consumer wash machines typically is about 60.degree. to 180.degree. F.
Institutional warewashing machines are generally either low temperature chemical sanitizing machines with a water temperature ranging from about 120.degree. to 140.degree. F., or high temperature machines with a water temperature of about 160.degree. to 180.degree. F. A low temperature warewashing system can be more desirable than a high temperature system because it avoids the heating expenses associated with the hotter water. In addition, it is much simpler to dispense a rinse aid in a low temperature system. A quantity of rinse water can be added to the sump of the automatic dishwashing machine in a low temperature system and circulated to rinse the dishes before draining. In such a system, the rinse aid need only be provided to the sump, and will function as the water circulates.
By contrast, in a high temperature warewash system, dissolved rinse aid is injected into the pressurized rinse water line prior to entering the machine and is then sprayed over the dishes through a rotating spray arm. A continuous stream of hot water is commonly provided through the spray arm for rinsing. Consequently, a rinse aid for use in a high temperature system must be dispensed into and sufficiently dissolved in the hot water stream against a back pressure before the water leaves the spray arm and contacts the dishes or other wares. This generally requires a more complex dispensing system.
One of the difficulties encountered in the use of a rinse aid is that the rinse cycle is typically the last cycle in the warewashing process, permitting solubilized particles of the rinse aid to remain on the clean dishes. Because of the possibility for ingestion of surface residual rinse aid, some countries require that such residue be of food grade quality.
Researchers have struggled with the problem of effectively applying and preserving solutions of solid rinse aid additives regardless of formulation type. Microbial activity is feared as a potential health risk, can result in biomass plugging of the dispensers, and can aesthetically deteriorate solid rinse aid products. A less than effective method has been to formulate into solid rinse aids relatively toxic preservative materials (e.g., formaldehyde, glutaraldehyde) to preserve the dispenser generated solutions from mold and bacterial growth. However, in some countries such as Japan, no toxic materials or preservatives are permitted for use in rinse additive compositions which can remain on food service wares once the washing process is complete.
Accordingly, a substantial need exists for an apparatus and method for preserving solid food grade rinse aids during the dispensing process without use of toxic preservative materials.